     [unlabelled-facet-mark x-labelled-facet-mark y-labelled-facet-mark xy-labelled-facet-mark]))
 
 (defn facet
-  "Returns a vega spec with a facetted layout based on the *facet-x*
+  "Takes a vega spec and produces a vega spec with a facetted layout based on the *facet-x*
   and *facet-y* fields."
   [inner-spec [datum & more :as data]]
   {:pre [(contains? datum *facet-x*) (contains? datum *facet-y*)]}

(def ^:dynamic *size* :size)
(def ^:dynamic *facet-x* :facet_x)
(def ^:dynamic *facet-y* :facet_y)

(def ^:dynamic *geopath* :geopath)

(defn- first-non-nil
  [data field]

                    :properties {:axis {:strokeWidth {:value 0}}}}]
            :marks facetted-marks})))



(defn assoc-hover
  [vg hover-spec]
